Back boxes on block before dot and dab

[chriselevate]

I’ve got some 25mm back boxes to replace before the plasterer fits 12.5mm plasterboard and skims. As the internal blocks are quite crumbly I’d rather not recesses the boxes but just screw the to the block.

Will the dot, dab and plaster go over the 25mm deep boxes ok?

Thanks

 

[Deleted member 174758]

I can't see the plasterer liking it much (based on my interactions with them over my years as a joiner) - they normally need to "bully" the boards into position to get it plumb and straight and they should be packing up off the floor at the bottom as well (prevents subsequent wicking of water in the event of a flood, etc). Doing this on a stud wall is a lot easier.

 

[chriselevate]

Sadly I don’t have stud walls in that room so having to fix to block work.

 

[Deleted member 174758]

Just wondering how the plasterer will manage to slide the board up to the adjoining one if he's working to millimeter precise openings. Why can't you do the dot and dab, then once the board is set cut out the opening(s) in the plasterboard with a multitool? That will reveal the blockwork for a back box to be attached to). There are specific plugs designed for use in even the softest blockwork

 

[chriselevate]

Surely the cable would get in the way stop the box from fitting in. It’d be almost impossible to miss the cable with the multi tool when cutting out the board for the box surely?

 

[foxhole]

I fitted my boxes first , offered up plasterboard ( usually insulated) and tapped over box to mark rear of board then trimmed out and secured. I used fixing foam which made it easier and timber below each board to keep tight to ceiling .With plain plasterboard I recessed box leaving just 5mm proud to mark the rear of board.

 

[chriselevate]

Were those 35mm boxes or 25mm?
I'm wondering whether or not I need to recess my 25mm ones.

 

[foxhole]

I always recess , never hurts to have extra room for wiring and face plate screws can be found to accommodate any depth.

 

[chriselevate]

What’s the best tool/technique todo this? If the blocks were good I’d drill a series of holes and chisel out but they are not.

Is there a multi tool blade that can do it for example?

 

[foxhole]

What type of block? Mine are thermolite so even a tea spoon would cut them.
Would lean toward a 4” grinder to do most of the work is blocks are solid .